Method For The Selective Catalytic Reduction Of Nitrogen Oxides In Combustion Flue Gases And System For Implementing It

ABSTRACT

The present invention relates to limiting emissions of nitrogen oxides into the environment. In particular, the present invention relates to an improved process for the catalytic reduction of nitrogen oxides to nitrogen

The present invention relates to limiting emissions of nitrogen oxides into the environment. In particular, the present invention relates to an improved process for the catalytic reduction of nitrogen oxides to nitrogen.

Nitrogen oxides, or NOx, mainly contain nitrogen monoxide (NO) and nitrogen dioxide (NO₂). In the atmosphere, NOx can combine with water and produce nitric acid HNO₃, which contributes, in particular, to acid rain, to photochemical smog, and may be responsible for certain respiratory diseases, such as asthma. For these reasons, the reduction of NOx emissions into the atmosphere has become a major issue in recent years and is the subject of more and more restrictive regulations for industrialists.

NOx are especially produced during the combustion of liquid or gaseous fuels, such as for example hydrocarbons, natural gas, refinery gas, hydrogen, or else mixtures thereof, and then escape into the atmosphere via the combustion flue gases.

Such combustions are especially used for heating, reforming or cracking hydrocarbon or non-hydrocarbon fluids in petrochemical process furnaces. The term “heating” fluids is understood to mean preheating and/or heating and/or vaporizing and/or superheating fluids. Among the fluids in question, mention will be made, in particular, of liquid and/or gaseous hydrocarbons, heat-transfer fluids for chemical or petrochemical processes and sometimes also water (preheating of demineralized water or of boiler feed water, steam generation, superheating of steam).

Such combustions are also used for achieving the high temperatures necessary for the steam reforming reactions of hydrocarbon feeds (SMR or steam methane reforming). Steam reforming consists of the dissociation of hydrocarbon molecules (especially methane, CH₄) in the presence of steam and heat (several hundreds of degrees), and is mainly used in industry for the production of high-purity hydrogen (H₂). Such a process is described, in particular, by Michael D. Briscoe in U.S. Pat. No. 6,749,829. The steam reforming process is carried out in a steam reforming furnace, supplied on the one hand with hydrocarbon feed and with steam, and on the other hand with heat. When the furnace is hot, the hydrocarbon feed is then injected, as a mixture with steam, into tubes that contain catalyst and that pass through the furnace. The high temperature of the furnace, several hundreds of degrees, maintained by virtue of the combustion, then enables, in the tubes, the dissociation reaction of the molecules of the hydrocarbon feed and the production of synthesis gas (syngas) which will then be treated. Examples of hydrocarbon feeds according to the invention are hydrocarbons, oils and natural gases. Preferably, the hydrocarbon feed is chosen from hydrocarbons that can be vaporized below 250° C. that are of various origins, especially of fossil and plant origin. Preferred hydrocarbon feeds are natural gas, naphtha, LPG (liquefied petroleum gas), butane, propane, biodiesel, bioethanol, refinery off-gases, and all off-gases in general.

For these two types of furnace, the heat is generally supplied by the combustion of various fuels with air. This combustion is carried out in the radiant section, sometimes also known as the combustion chamber, by virtue of burners, positioned in the top and/or in the bottom and/or on the side walls of the radiant section.

The flue gases, combustion products resulting from burnt fuels, are themselves discharged at the outlet of the radiant section, via at least one flue gas duct, through the convection section, where they are cooled before their release into the atmosphere. Frequently, in order to reduce the fuel requirements intended to be burnt in the furnace, a system of preheating the combustion air is installed upstream of the burners. Thus, in order to improve the thermal efficiency of the furnace, the combustion air is preheated by recovering a portion of the heat available in the flue gases passing through the convection section, this being by means of heat exchangers of the plate heat exchanger or tubular heat exchanger type for example, these exchangers are also known as air preheaters.

However, as mentioned previously, these processes generate NOx in the combustion flue gases at the outlet of the convection section. Produced during the combustion of the fuels in the presence of air, they are released into the atmosphere with the combustion flue gases. Furthermore, the use of preheated combustion air results in the increasing of the NOx concentration of the flue gases due to the increasing of the flame temperature.

Several processes have been developed in order to reduce the concentration of the NOx present in the combustion flue gases. Thus, recourse may especially be had to a process of selective catalytic reduction (SCR) of the content of NOx in the flue gases. In such a process, a hot air/ammonia mixture (mixture also known as AAM) is injected, via an injector, into the convection section. The term ammonia should be understood in this context to mean aqueous products containing ammonia; it could be, for example, carbamide or urea. The mixture constituted by the combustion flue gases and the AAM is then brought into contact with a catalyst, within a suitable temperature range, from 200° C. to 600° C., that makes it possible to convert the NOx to nitrogen (N₂) and thus to reduce the NOx concentration of the flue gases released into the atmosphere. Such processes are described, in particular, in U.S. Pat. No. 5,612,010, U.S. Pat. No. 5,401,478 and U.S. Pat. No. 6,361,754.

In the known processes of selective catalytic reduction, hot air is mixed with aqueous ammonia in order to form the AAM. In a known manner, this hot air originates from fresh air which passes through a fan and is then heated, via a preheater outside of the convection section and of the flue gas duct; another known, but less used, solution consists in replacing the hot air with combustion flue gases withdrawn directly from the flue gas duct of the convection section, via a dedicated fan, downstream of the SCR. The AAM is then injected via an injection grid into the convection section, upstream of the SCR catalyst.

However, the efficiency of this process of selective catalytic reduction of the nitrogen oxides is obtained by means of a supplementary consumption of energy, especially electrical or steam energy, mainly necessary in order to heat the fresh air, or to a lesser extent the flue gases, used to produce the AAM mixture, and also for feeding the fan with fresh air or flue gases.

This energy overconsumption represents a significant cost for industrialists.

There is therefore a need for an improved selective catalytic reduction process that consumes little energy, which allows the selective catalytic reduction of nitrogen oxides present in combustion flue gases.

The nitrogen oxide reduction process (also known as P_(NOx) process in the remainder of the description of the invention) can be applied, in particular, to furnaces for carrying out petrochemical processes and to furnaces that carry out a steam reforming process.

In the remainder of the description, the term “PI process” will represent either (except for particular cases that are obvious on reading the text) a reforming process carried out in a steam reforming furnace or a process carried out in a petrochemical process furnace.

The present invention responds to the problem identified above by virtue of the use of an improved P_(NOx) process in particular that consumes less energy and that has a lower investment cost than the processes known for reducing the NOx concentration of the combustion flue gases of a PI process furnace, for a comparable efficiency.

One subject of the invention is a PI process for treating a feed in a furnace, the process comprising at least:

-   -   a step of releasing heat via the combustion of a liquid or         gaseous fuel in the presence of air preheated by at least one         air preheat circuit passing through the convection section of         the flue gases one or more times, this step resulting in the         creation of nitrogen oxides in the flue gases, which flue gases         are discharged into a flue gas duct through the convection         section located downstream of the radiant section (combustion         chamber) of said furnace; and     -   a step of selective catalytic reduction of said nitrogen oxides         of the flue gases, comprising the injection, in the convection         section in which at least one catalyst is present, of a mixture         of hot air and of ammonia, characterized in that the hot air of         said mixture is directly withdrawn from said at least one air         preheat circuit being used to feed the furnace with preheated         air.

In one preferred variant of the invention, the PI process is a preheating and/or heating and/or vaporization and/or superheating treatment of hydrocarbon or nonhydrocarbon fluids, in a petrochemical process furnace.

According to another preferred variant of the invention, it relates to a PI process for reforming a hydrocarbon feed in the presence of steam in a steam reforming furnace.

The furnace for implementation of the PI process according to the present invention may correspond to any furnace used in practice by a person skilled in the art. The furnace is supplied by any available means with fuel and with hot air. Generally, the furnace is combined with a piping system that makes it possible to introduce each fluid and each reactant within it.

The heat provided to the furnace (SMR or petrochemical furnace) in order to operate is preferably obtained by virtue of burners, situated, in particular, against the walls of the furnace, in particular against the side walls of the furnace, and/or on the floor and/or on the ceiling of the furnace. These burners make it possible to burn a fuel in the presence of air in order to reach the high temperatures needed for the various operations.

The term “fuel” within the meaning of the invention is understood to mean any liquid or gaseous fuel, especially fossil fuels, hydrocarbons, off-gases of petrochemical processes which may comprise gases such as methane and other gaseous hydrocarbons, but also carbon monoxide and hydrogen, and also all the mixtures of these fuels in the presence of other combustible or non-combustible fluids (H₂O, CO₂, N₂, etc.).

The furnace used in the present invention may correspond to any furnace used in practice by a person skilled in the art. The furnace is supplied via any available means with feed (hydrocarbon or other feed), with steam and with heat (hot air).

In order to reduce the energy consumption, the combustion air which feeds the burners of the furnace is preheated before it arrives in the furnace, via at least one air preheat circuit, sometimes two or even three or more. These are preferably ducts located in contact with the combustion section, along the convection section, and that are capable of passing through said convection section, at least once, preferably several times. The air preheat circuits are preferably supplied with so-called fresh air by fans which withdraw air from outside of the installation and inject it into the preheat duct. Thus, fresh air circulates in the duct at the air preheaters, a heat exchange occurs between the air and the hot combustion flue gases exiting the radiant section of the furnace. During each crossing of the convection section, the air is preheated whilst the combustion flue gases are cooled. The heat exchanges will be even greater when the air preheat circuit crosses the convection section of the combustion flue gases a high number of times.

The invention also relates to a PI process as described previously, in which the at least one combustion air preheat circuit is supplied with fresh air using at least one fan.

In one particular embodiment, the process of the invention is characterized in that the at least one combustion air preheat circuit crosses the convection section at least once, preferably at least twice (through an air preheater in order to be preheated therein and in order to cool the flue gases). In other embodiments, the at least one air preheat circuit may cross the convection section of the flue gases on three occasions, four occasions or even five occasions, a person skilled in the art being in a position to adapt the process as a function of the preheating necessary and of the size of the installation used.

Preheaters may cross the convection section of the flue gases downstream or upstream of the injection site of the AAM mixture of hot air and of ammonia-based product.

The process according to the invention provides a step of selective catalytic reduction of the nitrogen oxides contained in the flue gases in order to reduce, or even eliminate, their presence in the flue gases released into the air.

The expression “selective catalytic reduction” within the meaning of the invention is understood to mean a reaction between a preferably aqueous, ammonia-based product and the NOx, in the presence of heat and of a catalyst, resulting in the formation of inoffensive nitrogen N₂, which is released into the atmosphere. The catalyst used may be any catalyst judged suitable by a person skilled in the art. Thus, according to the process of the invention, a mixture of an aqueous ammonia-based product and of hot air originating from the at least one preheat circuit according to the invention is produced, this AAM mixture according to the invention being injected directly into the convection section. The mixture thus injected comes into contact with the catalyst present in the convection section; the selective catalytic reduction of the NOx present in the flue gases then takes place.

The process according to the invention makes it possible to significantly reduce the energy consumption of the furnace, it also makes it possible to reduce the number of pieces of equipment necessary for supplying hot air to the mixer. Indeed, by virtue of the fan that injects fresh air into the ducts at the start of the preheat circuit and that ensures the circulation of the air along the circuit, the air circulating in the preheat circuits is at a higher pressure than that of the furnace, whether this is in the radiant section or in the convection section, this being in order to compensate for the pressure loss in the burners.

Consequently, the fraction of preheated air of the preheat circuits withdrawn to form the AAM arrives directly at the mixer, the solution of the invention makes it possible not only to eliminate the air fan necessary for supplying the mixer with hot air but also to eliminate the dedicated fresh air preheater.

In one particular embodiment of the invention, the process as described previously is characterized in that the injection of the mixture of hot air and of ammonia (more generally of ammonia-based compound) takes place in at least one location in the convection section of the flue gases, upstream of the catalyst, which is held in place in the convection section by a suitable support.

In other embodiments in accordance with the invention, the injection of AAM may take place in several locations of the convection section, upstream of the catalyst, the number of injection locations being directly linked to the size of the installation used.

The AAM injection location may in particular be situated between two air preheaters passing through the convection section.

In one particular embodiment, the PI process as described previously is characterized in that the furnace is supplied with preheated air via a single air preheat circuit, and in that the hot air used during the step of selective catalytic reduction of the nitrogen oxides is withdrawn directly from said single air preheat circuit.

In another particular embodiment, the process as described previously is characterized in that the furnace is supplied with preheated air via two air preheat circuits, and that the hot air used during the step of selective catalytic reduction of the nitrogen oxides is withdrawn directly from at least one of said two air preheat circuits.

When the air preheat circuit(s) passes through the convection section of the flue gases at least once, preferably at least twice, the air preheat circuit comprises one, two, or even more air preheaters, situated in the convection section of the flue gases. Thus, the hot air used during the step of selective catalytic reduction of the nitrogen oxides is withdrawn directly from the air preheat circuit(s), either at the outlet of the first passage of said at least one air preheat circuit in the convection section of the flue gases, or at the outlet of the second passage of said at least one air preheat circuit in the convection section of the flue gases, or at the outlets both of the first and of the second passage of said at least one air preheat circuit in the convection section of the flue gases.

According to a second aspect of the present invention, this also relates to an installation for treating a feed in a process furnace comprising at least:

-   -   a PI process furnace that enables the treatment of a feed,         comprising means of feeding with fuel, and with air;     -   an air preheat circuit intended for supplying the furnace with         preheated air passing through the convection section of the flue         gases one or more times;     -   a convection section resulting in the combustion flue gases         produced during the combustion of fuels in the process furnace,         in which at least one catalyst for the selective reduction of         nitrogen oxides is present, preferably held in said section         using a support means;     -   a hot air/ammonia-based product mixer supplied with hot air by         said at least one air preheat circuit; and     -   an injector connected to said mixer, enabling the injection of         said mixture of hot air and of ammonia-based product into said         duct of the combustion flue gases, preferably upstream of the         catalyst, either directly upstream of the catalyst or further         upstream of the catalyst.

According to the invention, the expression “means of feeding” is understood to mean any system of pipework, ducts, valves, dampers, that make it possible to feed the process furnace (also called the furnace) with fuel and with preheated air.

According to the invention, the means for supplying the furnace with preheated air may consist of one, two, three or more air preheat circuits. Thus, the hot air supplying the at least one hot air/ammonia-based product mixer (also called the mixer) may be withdrawn from one, two, three or more air preheat circuits, depending on the installation used. The air preheat circuits according to the invention are supplied with fresh air by at least one fan. In this manner, as indicated previously, a saving is made on air fans necessary in the known installations for conveying hot air to the at least one mixer, that is to say an energy saving, a saving in the cost of the installation, and a saving of space.

The invention therefore also relates to an installation as described previously, characterized in that the at least one air preheat circuit is supplied with fresh air using at least one fan.

Still in accordance with the invention, at least one catalyst is present in the convection section of the furnace. The role of this catalyst is to accelerate the selective catalytic reduction of the NOx contained in the combustion flue gases. This catalyst is positioned inside the duct of the flue gases, preferably held using a support, and preferably occupying the entire cross section of the duct, so that all of the combustion flue gases pass through it.

Preferably, the catalyst is placed in the convection section in order to have air preheaters placed upstream and downstream, a person skilled in the art being qualified to adapt this arrangement so as to obtain a temperature of the combustion flue gases that is favorable to the selective catalytic reduction reaction.

Furthermore, according to the invention, the flue gas duct that transfers the flue gases from the radiant section to the convection section may be connected to the furnace at any level, that is to say at the top, at the bottom, in the middle or at any other level of the radiant section of the furnace, or even at several points of the radiant section of the furnace especially by means of several flue gas ducts that may then connect up into one and the same duct, so that the flue gases can be withdrawn as effectively as possible. A person skilled in the art is competent to adapt the architecture of the furnace and in particular the location of connection of the flue gas duct(s) where he desires the flue gases to be withdrawn.

In one particular installation according to the invention, the furnace is fed with preheated air via a single air preheat circuit. In this embodiment, the hot air feeding the at least one mixer is withdrawn directly from said single air preheat circuit.

In another particular installation according to the invention, the furnace is fed with preheated air by two air preheat circuits. In this embodiment, the hot air feeding the at least one hot air/ammonia mixer is withdrawn directly from at least one of said two air preheat circuits. In one particular installation according to the invention, said hot air is withdrawn from a single one of the two air preheat circuits. In another particular installation according to the invention, said hot air is withdrawn from the two air preheat circuits.

According to one preferred embodiment of the invention, the installation is characterized in that the at least one air preheat circuit passes through the flue gas duct in the convection section at least once, preferably at least twice. In this embodiment, the at least one hot air/ammonia mixer is supplied with hot air via a withdrawal from at least one air preheat circuit, either at the outlet of the first passage of the preheat circuit in the convection section of the flue gases, or at the outlet of the second passage of the preheat circuit in the convection section, or both at the outlet of the first and of the second passage of the preheat circuit in the convection section.

The term “mixer” within the meaning of the invention is understood to mean any device that makes it possible to homogeneously mix the hot air with the ammonia-based product. Such a device may consist of a static piece of equipment (in particular injection nozzle, baffle) or a dynamic piece of equipment (in particular propeller, rotor, blade). A person skilled in the art is able to adapt the number of mixers that he needs to the size of the installation and to the amount of flue gases produced. Thus, small installations will comprise only one mixer, whilst larger ones will comprise two, three, four, or even more thereof, each of the mixers being fed with hot air as described previously.

Similarly, in the installations according to the invention, the number of hot air/ammonia mixture injectors may also be varied by him as a function of the size of the installation and of the number of mixers that it uses.

In one particular embodiment, the injector may consist of an injection grid that enables a homogeneous injection of the AAM into the convection duct.

Preferably, the injector is positioned upstream of the catalyst in the convection section. In the part of the convection section located between the injector and the catalyst, a person skilled in the art may make provision for passing, or not, a water or air preheat circuit, or any other duct enabling the circulation of a heat-transfer fluid. In such an embodiment, the AAM injected into the convection section may thus, in turn, participate in a preheating.

In one particular embodiment of the invention, the mixture is produced from ammonia stored in tanks away from the installation, and sent to the mixer via an entrainment means, in particular by at least one pump. Optionally, the ammonia may be vaporized before it arrives at the mixer so as to facilitate its mixing with the hot air.

Optionally, the installation according to the invention also comprises a means for extracting combustion flue gases that are cooled and purified of nitrogen oxides. Such an extraction means may especially consist of a fan, located at the very end of the convection section, that makes it possible to discharge the flue gases into a stack that is open to the atmosphere.

The flue gases at the end of the convection section may also be sent directly into the stack, the stack being located, in this case, advantageously above the convection section.

According to one preferred variant of the invention, this relates to an installation in which the furnace is a petrochemical process furnace capable of preheating and/or heating and/or vaporizing and/or superheating petrochemical process fluids.

According to another variant of the invention, this relates to an installation in which the furnace is a steam reforming furnace capable of reforming a hydrocarbon feed.

Among the advantages presented by the invention compared to the known processes and installations, mention will be made of the following points.

-   -   Since the air withdrawn is already preheated and pressurized by         virtue of the fresh air fan feeding the air preheat circuit, it         may thus directly join the mixer without supplying supplementary         energy, the invention therefore enables the saving of a system         of heating air for supplying the at least one hot air/ammonia         mixer and also of fans supplying the preheaters with fresh air;         this saving represents both an energy saving and a saving of         space.     -   The use of hot air withdrawn from the combustion air preheat         circuit avoids the withdrawal of hot flue gases directly from         the convection duct of the flue gases, the latter being capable         of containing sulfur dioxide, and therefore of polluting and/or         corroding the mixer(s).

The present invention will be better understood with the aid of the detailed description of particular embodiments of the invention, given by way of illustration and non-limitingly, and illustrated using FIGS. 1 to 7, in which:

FIG. 1 and FIG. 2 represent diagrams of installations of the prior art;

FIGS. 3 to 7 represent diagrams of installations according to the invention.

The type of process used in the furnace is not specified, it could be a reforming process or a petrochemical process. The flue gas duct as represented in all of FIGS. 1 to 7 is connected to the furnace in the middle of the radiant section, it could be connected at any level; a person skilled in the art is competent to adapt the architecture of the furnace.

FIG. 1 describes an installation as used in the prior art, comprising a furnace 1 enabling the treatment of a feed 2 and the production of a product 3, that is fed with fuel 5 and with preheated air 6 via an air preheat circuit 7, which is fed with fresh air 8 by a fan 9. In this type of installation, the radiant section or combustion chamber 4 is connected to a convection section 10 that enables the discharge of the combustion flue gases 11 produced in the radiant section 4 during the combustion, said combustion flue gases 11 comprising, inter alia, nitrogen oxides. The fresh air is injected by virtue of the fan 9 into the air preheat circuit 7, which passes through the convection section 10 on two occasions with the exchangers (air preheaters) 24, 26. Thus, the fresh air 8 is heated as it progresses through the preheat circuit 7, whilst the combustion flue gases 11 circulating in the convection section 10 are cooled. The installation also comprises a mixer 12 of hot air 13 and of ammonia 14. On the one hand, the ammonia is stored in a tank 15 connected to a pump 16 that supplies the mixer 12 with ammonia 14. On the other hand, the hot air 13 originates from a reheater 17 supplied with fresh air 8 by a fan 18. Said mixer 12 is combined with an injection grid 19 enabling the injection of said mixture of hot air 13 and of ammonia 14 into the combustion flue gases 11 at the level of said at least one convection section 10 comprising at least one catalyst 20. In this embodiment, the injection grid 19 is placed in the convection section 10 upstream of the catalyst 20. Finally, the combustion flue gases that are cooled and purified of nitrogen oxides are expelled into a stack that is open to the atmosphere, with the optional aid of a flue gas extractor fan 21.

FIG. 2 also describes an installation according to the prior art, identical in every respect to FIG. 1, except that the hot air 13 feeding the mixer 12 is replaced by combustion flue gases 11 withdrawn from the convection section 10, with the aid of a fan 23 and after the catalyst 20.

FIG. 3 describes an installation in accordance with the invention, in which the references are identical to those of FIG. 1, except that the hot air 13 feeding the mixer 12 is directly withdrawn from the air preheat circuit 7, at the outlet of the first air preheater 24, via a duct 25 that directly connects said preheat circuit 7 to said mixer 12. In this particular embodiment, the temperature of the air withdrawn at the outlet of the first air preheater is generally between 100 and 400° C.

FIG. 4 describes an installation in accordance with the invention, in which the references are identical to those of FIG. 1, except that the hot air 13 feeding the mixer 12 is directly withdrawn from the air preheat circuit 7, at the outlet of the air preheater 26, via a duct 27 that directly connects said preheat circuit 7 to said mixer 12. In this particular embodiment, the temperature of the air withdrawn at the outlet of the second air preheater is generally between 250 and 600° C.

FIG. 5 describes an installation in accordance with the invention, in which the references are identical to those of FIG. 1, except that the hot air 13 feeding the mixer 12 is directly withdrawn from the air preheat circuit 7, both at the outlet of the first air preheater 24, and at the outlet of the second air preheater 26.

FIG. 6 describes an installation in accordance with the invention, identical to the installation of FIG. 4, except that the injector 19 is placed downstream of an exchanger 28 (commonly referred to as a convection bundle) the purpose of which is the preheating and/or the heating and/or the vaporization and/or the superheating of a fluid other than the combustion air, more specifically the injector 19 is placed between the exchanger 28 and the catalyst 20.

FIG. 7 describes an installation identical to the installation from FIG. 4, except that the injector 19 is placed upstream of an exchanger (convection bundle) 28.

A person skilled in the art may, according to another alternative (not represented), eliminate the fresh air fan 18 for the process of selective catalytic reduction of NOx when the combustion air used for the combustion in the radiant section is not preheated through the convection section and may use the fan 9 both for conveying fresh air to the radiant section (such as combustion air) and to the mixer 12 of the process of selective catalytic reduction of NOx. In this case, use will preferably be made of a preheater on the air duct used for the selective catalytic reduction. 

1-15. (canceled)
 16. A process for treating a feed in a furnace comprising: at least one air preheat circuit, a radiant section, a convection section located downstream of said radiant section, and a flue gas duct that passes through said convection section and in which said air preheat circuit is located, said process comprising: releasing heat via the combustion of a liquid or gaseous fuel in the presence of air preheated in said at least one air preheat circuit, wherein said preheat circuit passes through the convection section one or more times, thereby resulting in the creation of a flue gas containing nitrogen, discharging said flue gases into said flue gas duct through the convection section; and reducing said nitrogen oxides in the flue gases by means of selective catalytic reduction, said selective catalytic reduction comprising the injection, in the convection section in which at least one catalyst is present, of a mixture of hot air and of ammonia, wherein the hot air of said mixture is directly withdrawn from said at least one air preheat circuit being used to feed the furnace with preheated air.
 17. The process of claim 16, wherein the furnace is supplied with preheated air via a single air preheat circuit, and that the hot air used during the selective catalytic reduction of the nitrogen oxides is withdrawn directly from said single air preheat circuit.
 18. The process of claim 16, wherein the furnace is supplied with preheated air via two air preheat circuits, and that the hot air used during the step of selective catalytic reduction of the nitrogen oxides is withdrawn directly from at least one of said two air preheat circuits.
 19. The process of claim 16, wherein the hot air used during the selective catalytic reduction of the nitrogen oxides is withdrawn directly from the at least one air preheat circuit, either at the outlet of the first passage of said at least one air preheat circuit in the convection section, or at the outlet of the second passage of said at least one air preheat circuit in the convection section of the flue gases, or at the outlets both of the first and of the second passage of said at least one air preheat circuit in the convection section.
 20. The process of claim 16, wherein the injection of the mixture of hot air and of ammonia takes place in at least one location of the convection section, upstream of the catalyst, which is held in the combustion flue gas duct by a support.
 21. The process of claim 16, wherein the at least one air preheat circuit is supplied with fresh air using at least one fan.
 22. The process of claim 16, in which the treatment of the feed is a preheating and/or heating and/or vaporization and/or superheating treatment of fluids in a petrochemical process furnace.
 23. The process of claim 16, in which the treatment of the feed is a reforming treatment of a hydrocarbon feed in the presence of steam in a steam reforming furnace.
 24. An installation for treating a feed in a process furnace comprising at least: a process furnace that enables the treatment of a feed, comprising means of feeding with fuel, and with preheated air; an air preheat circuit that supplies the furnace with preheated combustion air passing through the convection section one or more times; a convection section resulting in the flue gases produced during the combustion of fuels in the radiant section of the process furnace, in which at least one catalyst for the selective reduction of nitrogen oxides is present, preferably held in said duct using a support means; a hot air/ammonia mixer supplied with hot air by said at least one air preheat circuit; and an injector connected to said mixer, enabling the injection of said mixture of hot air and of ammonia into the flue gases in said convection section, preferably upstream of the catalyst.
 25. The installation of claim 24, wherein the process furnace is supplied with preheated air via a single air preheat circuit, and that the hot air supplying the at least one hot air/ammonia mixer is withdrawn directly from said single air preheat circuit.
 26. The installation of claim 24, wherein the process furnace is supplied with preheated air via two air preheat circuits, and that the hot air supplying the at least one hot air/ammonia mixer is withdrawn directly from at least one of said two air preheat circuits.
 27. The installation of claim 24, wherein the at least one hot air/ammonia mixer is supplied with hot air via a withdrawal from the air preheat circuit(s), either at the outlet of the first passage of the preheat circuit in the convection section, or at the outlet of the second passage of the preheat circuit in the convection section of the flue gases, or both at the outlet of the first and of the second passage of the preheat circuit in the convection section.
 28. The installation of claim 24, wherein the at least one air preheat circuit is supplied with fresh air using at least one fan.
 29. The installation as of claim 24, in which the furnace is a petrochemical process furnace capable of preheating and/or heating and/or vaporizing and/or superheating petrochemical process fluids.
 30. The installation of claim 24, in which the furnace is a steam reforming furnace capable of reforming a hydrocarbon feed. 